Last year, I put off a full antivirus scan because it said it would take eight hours. I figured I could do it later, but later never came. A week after, my bank details were stolen from a keylogger that the scan would have caught. Now I schedule scans during my sleep, no excuses.
Ugh, that is such a rough way to learn that lesson. It's crazy how one skipped scan can cause so much real trouble. Setting it up to run automatically is the only way to go.
My cousin's old laptop got so slow after missing scans, it felt like a 1998 dial-up modem. @theamartinez has the right idea with weekly runs, saves so much headache.